In order to improve the shock absorption properties of running shoes it has become conventional to incorporate relatively thick and soft foam materials into the heel of the shoe so as to improve the cushioning effect of the shoe. However, such softer cushioning of the heel has provided less firm support to the heel and so has contributed directly to the problems of excessive rear-foot movement or instability. Good shock aborption and rear-foot movement control thus give rise to conflicting requirements.
For control of rear-foot movement it is required to restrain movement inside the shoe during running so as to limit the degree of pronation and/or supination of the foot. Pronation is the inward rotation of the foot about a horizontal axis parallel to the direction of motion, and supination is the outward rotation of the foot about the same axis. The human anatomy is such that when most people run, or walk for that matter, the foot initially contacts the ground during each step on the outside (valgus) edge of the sole somewhere between the heel and ball of the foot and the foot is supinated. As the weight on the foot increases so the ankle rolls or rotates inwards into a flat, stable position, passing through "neutral" to a slightly pronated position where the body weight of the runner is brought from the outside edge of the foot inwardly towards the center of the arch. The foot then rolls outwardly and toes off towards the great toe to complete the step. Pronation is thus a normal and essential function of the foot and it is only when it becomes excessive that problems can arise. Over-pronation occurs when the inward rotation of the foot continues to an excessive degree such that the weight is transferred to the inside (varus) edge of the sole.
It has been recognised that the vast majority of runners exhibit substantially different degrees of pronation when running in prior art shoes. Many people initially land on the outside edge of the heel of the ground contacting foot, with the foot rolling inwards with improper biomechanical action and remaining rolled inwards. Such improper action comprises overpronation. It has been established that, when running, three to four times the gravitational force is applied to the outside edge of the foot, and in particular to the heel area, than occurs during walking; this high force concentrated at the heel area, coupled with a tendency of most runners to roll their feet inwards forcefully during running, results in increased stressing of the foot and leg and increased heel wear of the shoe being worn, and as the shoe wears and its rear-foot support capabilities break down the problems are increased.
Shoes have been proposed, for example in FR-A-2 396 524, in which the sole of the shoe in the region of the heel is of non-uniform thickness between the inner (varus) side of the shoe and the outer (valgus) side of the shoe and, to similar effect, it has been proposed, for example in FR-A-2 522 482, to incorporate a transverse wedge of harder foam plastic material within the softer foam plastic material of the heel to reduce the amount of heel deformation at the inside edge. Yet a further proposal is disclosed in U.S. Pat. No. 4,235,026 and comprises the provision in an elastomeric shoe sole designed specifically for athletic activities of a plurality of transversely extending, longitudinally spaced openings at the outer side of the heel and extending to approximately the longitudinal center line of the shoe, and with the opposite side of the heel substantially solid, thereby allowing the sole to yield at the outer side of the heel to a greater extent than at the inner side. None of these prior art proposals has given any consideration to the different requirements of the individual users as regards their weight and running style, or to the fact that the shoes may be used under differing running conditions as for example different ground surfaces, and scant attention has been directed in the prior art to the restraint of rear-foot movement.
Consideration has been given in the prior art to the question of adapting the shock-absorbing capabilities of running shoes to the individual requirements of the user, and in No. DE-A-2 904 540 and U.S. Pat. No. 4,430,810 there is described an arrangement wherein a number of bores extend through the relatively soft material of the heel portion of a running shoe from one side thereof to the other, with the bores being spaced apart from each other in the longitudinal heel-to-toe direction of the shoe, and rod-shaped stiffening members of selectable greater hardness than the soft heel material can be inserted into the bores so as selectively to increase the overall hardness of the sole and adapt the shock-absorbing capabilities of the shoe to the individual requirements of the runner and to the nature of the surface upon which he intends to run. As described in DE-A-2 904 540, the heel itself can be made of a plastic foam material having a comparatively low Shore hardness of only about 35, and the supporting members can be made of all possible materials such as PVC, polyethylene, polyamide, nylon and even of metal. The proposal to stiffen the heel of a shoe by insertion of appropriate stiffening elements into bores in the heel is known also from FR-A-958 766, and in U.S. Pat. No. 3,785,646 there is disclosed a shoe having a rubber sole with transverse bores into which rod-like metal weights may be inserted. In none of these documents is any consideration whatsoever given to the question of control of rear-foot movement.
There is further described in DE-GM-8335315 (which was filed on Dec. 9, 1983, namely before the earliest priority date of the present application, but was not registered until Sept. 20, 1984 and was not made available until Oct. 31, 1984, namely after the latest of the priority dates of the present application) a sole arrangement for sports shoes which is a variation of the arrangement described in DE-A-2 904 540 designed to enable variable heel cushioning to be achieved even at the rear edge of the heel where the transverse stiffening arrangement of DE-A-2 904 540 cannot be used. In accordance with the proposal of DE-GM-8335315, a longitudinal recess is provided in the relatively soft heel material and extends from the rear edge of the heel for receiving a longitudinal support element of selectable hardness, and a transverse recess extending between the inner and outer edges of the heel intersects the longitudinal recess for receiving a pair of transverse support elements, each of selectable hardness which interlock with the longitudinal support element on opposite sides thereof. Even in this arrangement, where it is said that each of the three support elements can have different qualities or features, the problems of rear-foot instability are not considered, and it is even suggested to incorporate a stiffener in the outer transverse support member which would exacerbate any problems of over-pronation.